It is known to provide as a primary containment for a nuclear reactor core, or, more generally, as a pressure vessel adapted to enclose a nuclear reactor, a cylindrical receptacle provided with a cover or top at a floor or bottom which are composed, together with the cylindrical vessel walls, of cast elements.
As described in the aforementioned application, such elements may be annular or ring elements, segments or sectors and can be stacked to form the cylindrical vessel wall and provided with a cast cover and bottom. The interior of the vessel may be provided with a welded or seamless tubular lining and the cover, base or walls may be formed with fittings communicating with the interior of the vessel to permit operation of the nuclear reactor therewithin.
The principal function of the vessel is to sustain the pressure which is developed within the reactor chamber and to contain any explosive or other sudden pressure increase. In the event of a catastrophe the function of the vessel is to minimize the energy dissipated to the environment.
As indicated, it is known to constitute such pressure vessels from cast elements such as elements composed of cast iron and steel and it is also known to stress or prestress or reinforce such elements with appropriately disposed stressing cables.
For example, the stacked elements may be provided with intercommunicating axial channels through which axial stressing cables can extend, these cables being placed under axial stress and being seated at their opposite ends against the base and cover of the unit.
In general, each "stressing cable" may consist of a multiplicity of individual cable elements in mutually parallel relationship or twisted together. Furthermore, a plurality of such cables can be angularly equispaced about the axis of the vessel and can extend parallel to each other and to the axis thereof through the aforementioned channels. Such cables can constitute the axially extending, axial-stress cables which run through the axial passages in the cast elements of the vessel wall.
In addition, the vessel is provided with peripheral stressing cables which extend circumferentially around the vessel in axially spaced relationship. These circumferentially stressed cables each may also consist of a plurality of cable elements and may have the ends of each cable drawn together by hydraulic or other means to apply the desired degree of inward or radial stress upon the vessel. While the stress may be applied to the external wall of the vessel directly, it can, as the aformentioned applications describe, be applied through shoes which are angularly spaced about the periphery of the vessel and which support the peripheral stressing cable.
It is customary to provide the total set of axial stressing cables from a certain number of normal-operating axial stress cables and a number of additional safety cables which also run parallel to the operating axial stress cables. The total set of axial-stress cables, therefore, is formed by the desired number of operating axial stress cables and the desired number of safety axial stress cables. The safety cables may be disposed between the operating cables. The difference between the operating cables and the safety cables can be negligible although it should be understood that the number of operating axial-stress cables is always equal to the number of cables required to sustain the vessel without distortion under its maximum operating pressure. In other words, the safety cables are required only in case the maximum safe operating pressure is exceeded.
Similarly, the actual number of peripheral stressing cables is made up of the desired number of operating peripheral-stress cables and the desired number of safety peripheral-stress cables, the relationship between the operating and safety peripheral stress cables being the same as that given above for the axial stress cables.
It is customary in connection with such pressure vessels to provide monitoring systems capable of alerting operating personnel to the potential development of catastrophic or disastrous or even unusual conditions. Naturally, the pressure vessel of this type can be provided with pressure sensors to monitor the rate of change in pressure, the pressure itself, the temperature and like internal operating parameters of the system which signal potentially dangerous conditions or even unusual conditions which can lead to malfunction of the reactor. Such monitoring systems are well known in the art and are intended to enable operating personnel to take corrective measures so as to ensure long term operation of the nuclear reactor in a safe manner.
The system, however, requires, for absolute safety, assurance that the mechanical elements of the pressure vessel are in operating shape. This too has been done in the past, generally by monitoring the temperature of the external portions of the vessel.
In other words, it is known to monitor the temperature of the pressure vessel of cast elements to endeavor to ensure that these structural elements will not be adversely effected by the reactor operation.
However, this is not always sufficient and frequently fails to provide sufficient warning of a structural defect in sufficient time to enable corrective measures to be taken.
Hence it is desirable to provide a system in which the mechanics or structural integrity of the system can be monitored more effectively or to a greater degree.
The monitoring of the mechanics or structural integrity of the system is intended here to mean, not the thermal monitoring in the sense described above, but rather monitoring of the bearing strength of the cast structural elements and the walls assembled therefrom, the monitoring of the bearing strength of the cover and base or floor of the pressure vessel, and the monitoring of the stability, strength or fluctuation of other characteristics of the stressing system consisting of the axial stressing and peripheral stressing cables.
The differentiation between normal-operation axial-stress cables and safety axial-stress cables on one hand and the operating peripheral-stress cables and safety-peripheral stress cables on the other, will be understood to relate to the operability of the pressure vessel. In other words, only the normal-operating stressing cables are required to make the pressure vessel fully functional during normal operation.
It is, however, customary to make both systems of stressing cables structurally and functionally interchangeable in the sense that there need be no structural differentiation between a normal-operating stressing cable and a corresponding safety stressing cable.
When, for example, eighteen axial-stressing cables are angularly equispaced about the periphery of the vessel, nine can be constituted as normal-operation stressing cables while nine are constituted safety cables. From a practical point of view it is of no significance whether the even or odd axial stressing cables are the normal-operating cables and the others are the safety cables or vice versa. The same applies for the peripheral stressing cables.
Notwithstanding the progress which has been made, as described above, in developing pressure-retentive structures to serve as nuclear reactor containments, there has been a need for a simple, reliable and long-lived system for detecting failures, defects or structural deficiencies (i.e. monitoring the mechanics) of such vessels to alert the operating personnel of potential danger with respect to the structural integrity of components of the vessel.
Such a monitoring system must be effective whether the deficiency or defect is a defect in the strength of the materials used, a defect in the structural element constituted from the material, a defect in the wall assembled from the cast elements, a defect in the case metal cover, a defect in the cast metal base, or a defect in the one of the cables or stressing elements and, in general, a failure anywhere in the stressing system.